The present invention relates to digital communications systems and, more particularly, to a novel transceiver for transmitting and receiving data by phase-shift-modulation of a communications system carrier waveform.
It is well-known that communications systems may transfer digital information from one location to another location by means of a carrier having the phase thereof shifted in accordance with the value of each of a sequence of data bits. Typically, a phase-shift modulation communications system may invert the phase of the carrier waveform, i.e. phase-shift modulation by 180.degree., during each bit time interval in which a first binary data value is to be transmitted, and maintain the carrier waveform phase of the immediately preceding bit time interval, when a remaining binary data value is to be sent in a particular bit time interval. It is also known that transmission of digital information may occur utilizing a powerline as the transmission medium between a central facility and a multiplicity of remote locations each tied to the power distribution network. Powerline communication systems of this type are disclosed and claimed in U.S. Pat. Nos. 3,973,087 and 3,973,240, both issued Aug. 3, 1976; U.S. Pat. No. 3,944,723, issued Mar. 16, 1976; U.S. Pat. No. 4,135,181, issued Jan. 16, 1979; and U.S. Pat. No. 4,161,720, issued July 17, 1979, all assigned to the assignee of the present application and all incorporated herein in their entirety by reference. In powerline communication systems, wherein a powerline frequency (typically 60 Hertz (Hz.) in the United States) signal is present throughout the entire system, it is also known that the signal-to-noise ratio of the received signal can be greatly improved if the transmitted carrier signal frequencies are odd multiples of the first sub-harmonic of the powerline frequency. Methods and apparatus for realizing such improvements in signal-to-noise ratio are described and claimed in U.S. Pat. No. 4,101,834, issued July 18, 1978; and U.S. Pat. No. 4,109,204, issued Aug. 22, 1978, both assigned to the assignee of the present invention and incorporated herein by reference in their entirety.
Present concepts for automatic meter reading and control systems, in which each power-consuming location (a residence, office building, factory and the like) can be controlled and monitored, require that a meter terminal unit (MTU) be capable of both receiving control information from a central location and of transmitting power consumption information back to the central location upon request. Thus, in addition to received-data-processing and sensed-data-formatting apparatus, each MTU will desirably include a transceiver capable of receiving the phase-shift-modulated carrier from the central location for recovery of control data and of transmitting a phase-shift-modulated carrier back to the central location for conveying data thereto. As the number of MTUs required for even a moderately sized power distribution network can number in the hundreds of thousands, a relatively low cost transceiver, having a reduced complexity, is highly desirable.